Short-circuit current limiter

ABSTRACT

A short circuit current limiter is disclosed which includes fuse elements configured in one of a folded back configuration or a switch back configuration, a plurality of contact terminals, one contact terminal electrically connected to each end of each fuse element, a terminal base for mechanically mounting the fuse elements and contact terminals, and a cover configured to engage the terminal base to protect the fuse elements.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to fuses, and, moreparticularly, to current limiters which provide short circuitprotection.

[0002] Fuses are widely used as overcurrent protection devices toprevent costly damage to electrical circuits. An overcurrent conditionmay be attributable to an overload current, that is, an excessivecurrent relative to a normal operating currents found in the electricalcircuit and confined to the conductive paths of the electrical circuit.Additionally, an overcurrent condition may constitute a short circuitcondition, or current flowing outside the normal conducting paths of theelectrical circuit. Short circuit currents are also sometimes referredto as fault currents.

[0003] Fuse terminals typically form an electrical connection between anelectrical power source and an electrical component or a combination ofcomponents arranged in an electrical circuit. One or more fusible linksor elements, or a fuse element assembly, is connected between the fuseterminals, so that when electrical current through the fuse exceeds apredetermined limit, the fusible elements melt and opens one or morecircuit through the fuses to prevent electrical component damage. Incertain applications, fuses will not open from overload currents due totemporary surges in drawn current occurring in, for example, starting ofmotors and energization of transformers. The temporary surges aretypically between one and six times a normal current level, and usuallywill not cause damage to components of the electrical circuit unless theoverload condition is sustained over a length of time. Fuses typicallydo not react to an overload current of a short duration.

[0004] Fuses also protect against short circuit currents, which can beas high as many hundreds of times larger than normal operating currents,and therefore require rapid isolation in order to protect the electricalcomponents.

[0005] Circuit breakers are also widely used as overcurrent protectiondevices to protect electrical circuits. While circuit breakers provideoverload protection, known mechanical circuit breakers provide limitedshort circuit interruption capability compared to fuses. Addingadditional fuse protection to achieve or extend a short circuitinterruption rating in a circuit which is already overload protectedusing circuit breakers, however, increases costs of circuit protectionand is typically not a space efficient solution.

[0006] An alternative solution to providing extended short circuitprotection capability in a circuit protected by circuit breaker is touse higher priced circuit breakers for the application that is beingprotected. For most customers, the prospect of having to purchase ahigher priced circuit breaker is an unattractive solution.

BRIEF SUMMARY OF THE INVENTION

[0007] In one aspect of the invention, a short circuit current limiteris provided which comprises at least one fuse element which includes afirst end and a second end. The fuse element incorporates one of afolded back configuration or a switch back configuration. The currentlimiter further comprises a plurality of contact terminals, one contactterminal electrically connected to each end of each fuse element, aterminal base for mechanically mounting the fuse elements and contactterminals, and a cover configured to engage the terminal base to protectthe fuse elements.

[0008] More specifically, in one embodiment, the short circuit currentlimiter includes three poles and respective visual indicators to providea status of corresponding fuse elements. Insulating barriers fabricatedfrom a fiberglass composite material, both line-to-load andphase-to-phase, to protect the fuse elements from shorting.Alternatively, phase-to-phase barriers are fabricated in the cover. Thefolded back and switch back configurations of the short circuit currentlimiter described herein provide a size advantage over known currentlimiters which employ conventional straight fuse element configurations.

[0009] The short circuit current limiter provides protection from shortcircuits and therefore allows use of less expensive circuit breakers toprovide overload protection in electrical circuit configurations. Acompact and cost effective current limiter package is provided forcombination with smaller sized, less expensive circuit breakers toprovide an adequate, affordable, and space-saving circuit protection forboth short circuit protection and overload protection. The short circuitcurrent limiter is also configured to be compatible with InternationalElectrotechnical Commission (IEC) style motor starter and motorcontactor installations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is an exploded view of a current limiter.

[0011]FIG. 2 is an exploded assembly view of the current limiter shownin FIG. 1.

[0012]FIG. 3 is a cross-sectional view of the current limiter shown inFIGS. 1 and 2.

[0013]FIG. 4 is another cross-sectional view of the current limitershown in FIGS. 1 and 2.

[0014]FIG. 5 is a partial assembly view of the current limiter shown inFIGS. 1-4.

[0015]FIG. 6 is a perspective view of an assembled current limiter shownin FIGS. 1-5.

[0016]FIG. 7 is an exploded view of a second embodiment of a currentlimiter.

[0017]FIG. 8 is a partial assembly view of the current limiter shown inFIG. 7.

[0018]FIG. 9 is a perspective view of a portion of the current limitershown in FIGS. 7 and 8.

[0019]FIG. 10 is a perspective view of an assembled current limitershown in FIGS. 7-9.

DETAILED DESCRIPTION OF THE INVENTION

[0020]FIG. 1 is an exploded view of an illustrative embodiment of ashort circuit current limiter 10 in which the benefits of the inventionare demonstrated. It is recognized, however, that current limiter 10 isbut one type of electrical component in which the benefits of theinvention may be appreciated. Thus, the description set forth below isfor illustrative purposes only, and it is contemplated that benefits ofthe invention accrue to other sizes and types of fuses and currentlimiters. Therefore, there is no intention to limit practice of theinventive concepts herein solely to the illustrative embodimentdescribed, that is short circuit current limiter 10.

[0021] Current limiter 10 includes a base 12 and a cover 14 which, in anexemplary embodiment, are fabricated from thermoset and thermoplasticmaterials according to known methods and techniques, including, but notlimited to, molding operations. A plurality of apertures 16 are formedthrough a top surface 18 of cover 14. Indicators 20 are configured to beinserted into cover 14 by insertion into apertures 16, and usuallyindicate a status of each pole of the current limiter. Current limiter10 is illustrated as a three pole device although the invention shouldnot be construed as being so limited, as fewer or additional poles couldbe employed in such a fusing device. In one embodiment, indicators 20are fabricated from a temperature sensitive material that appears whitewhen current limiter 10 is intact, i.e., operative or unopened. Upon ashort circuit condition, fuse elements 22 heat and eventually melt,opening the circuit, and the associated heat causes indicators 20 toturn black. In one exemplary embodiment, the primary material inindicator 20 is a micro-crystalline organic wax applied to a blackcolored substrate. In alternative embodiments, other color schemes maybe employed to indicate current limiter status, and in furtherembodiments, other known indicator mechanisms may be employed in lieu ofindicators 20.

[0022] Line-to-load insulating barriers 24 for each fuse element 22 areused to protect fuse elements 22 from shorting. Fuse elements 22 employa folded back configuration to reduce size of current limiter 10. Morespecifically, in the illustrated embodiment, fuse elements 22 aresubstantially U-shaped and includes first and second portions extendingsubstantially parallel to one another, and a third portion extendingsubstantially perpendicular to and joining the first and secondsections. In alternative embodiments, other fuse element configurationsare employed to reduce a size of current limiter 10 without reducinglineal length of fuse elements 22. In addition, fuse elements 22 includea number of constrictions or holes to reduce a cross-sectional area ofthe fuse elements so that the fuse elements melt, disintegrate, orotherwise open at predetermined current levels dependent upon fuseelement dimensions and characteristics.

[0023] Insulating barriers 24 are configured with tabs 26 which areinserted into slots 28 formed in base 12, for ease in assembling currentlimiter 10 and to support insulating barriers 24. In one specificembodiment, insulating barriers 24 are fabricated from a fiberglasscomposite material.

[0024] Fuse elements 22 include slots 30 which receive protrusions 32 oninsulating barriers 24 as fuse elements 22 are mounted on base 12.Contact terminals 34 are inserted into openings 36 in base 12 untilmounting rests 38 engage surface 40 of base 12, at which time a backsurface 42 of contact terminals 34 is supported against a supportsurface 44 of base 12. Once in place, contact terminals 34 areelectrically connected to fuse elements 22. The electrical connection isaccomplished when tabs 46 of contact terminals 34 are engaged by crimps48 which are formed into a first end 50 and a second end 52 of fuseelements 22. Additionally, crimps 48 and tabs 46 are electrically bondedvia known methods and techniques, such as a welding or solderingprocess, and may be bonded before insertion into base 12.

[0025] Fasteners 54 are inserted through attachment openings 56 formedinto base 12 and are used to attach cover 14 to base 12 at coverattachment points 58. Plugs 60 are inserted into filling holes (notshown) in a bottom 62 of base 12 after the current limiter 10 has beenfilled with an arc quenching media (not shown), such as quartz silicasand, in an exemplary embodiment, which absorbs arc energy when fuseelement 22 opens.

[0026]FIG. 2 is a partially assembled view of current limiter 10illustrating fuse elements 22, insulating barriers 24 and contactterminals 34 inserted into base 12. Fasteners 54 are also shown insertedinto base 12. Arc quenching media filling holes 70 extend through bottom62 of base 12. Referring to cover 14, attachment points 58 extend fromcover 12 for engagement with fasteners 54 to attach base 12 to cover 14.In alternative embodiments, other attachment methods are employed,including, but not limited to, riveting and ultrasonic weldingprocesses.

[0027] Cover 14 includes a plurality of chambers 72 defined byinsulating phase-to-phase barriers 74. Phase-to-phase barriers 74prevent fuse elements 22 from shorting when cover 14 is installed ontobase 12, since when installed, each individual chamber 72 houses asingle fuse element 22.

[0028]FIG. 3 is a side cross-sectional view of an assembled currentlimiter 10 including indicators 20 inserted through cover 14 and furtherincluding fuse elements 22 engaged with protrusions 32 of line-to-loadinsulating barriers 24. Contact terminal mounting rests 38 engagesurface 40 of base 12. Tabs 46 engage crimps 48 molded into first end 50and second end (shown in FIG. 1) of fuse elements 22 to establish anelectrical connection of contact terminals 34 to fuse elements 22.Phase-to-phase barriers 74 prevent fuse elements 22 from shorting to oneanother when cover 14 is installed onto base 12, providing each fuseelement 22 with an individual chamber 72.

[0029]FIG. 4 is an end cross-sectional view of an assembled currentlimiter 10. Current limiter 10 includes indicators 20 inserted intocover 14 and further includes folded back fuse elements 22 inserted ontoprotrusions 32 of line-to-load insulating barriers 24. Insulatingbarriers 24 prevent fuse elements 22 from short circuiting, therebycompromising the short circuit detection capability of current limiter10. Insulating barrier 24 and fuse element 22 are positioned within achamber 72 when cover 14 is attached to base 12. Contact terminalmounting rests 38 engage, or rest upon surface 40 of base 12. Mountingrests 38 of contact terminals 34 rest upon surface 40 of base 12 whencontact terminals 34 are inserted into base 12 as previously described.

[0030]FIG. 5 is another partially assembled view of current limiter 10including protrusions 32 of insulating barriers 24 inserted into slots30 (shown in FIG. 1) of fuse elements 22. Crimps 48 at first ends 50 offuse elements 22 engage tabs 46 of contact terminals 34, and contactterminal mounting rests 38 rest on surface 40 of base 12 after insertionof contact terminals 34 into molded openings 36 of base 12. Fasteners 54are inserted into attachment holes 56 (shown in FIG. 1) of base 12.

[0031]FIG. 6 is an assembled view of current limiter 10 showingindicators 20 inserted into cover 14 and contact terminals 34 asinstalled into base 12. Current limiter 10 is configured to protectelectrical equipment and circuit breakers from short circuit currents,which are many times larger than normal operating currents. Further,current limiter 10 provides the rapid isolation necessary to protect theelectrical circuits from such excessive currents. To provide suchprotection, current limiter 10 is inserted in an electrical circuitwhere one or more of contact terminals 34 connect to an electrical powersource and other contact terminals 34 connect to an electrical componentor a combination of components, thereby providing an electrical currentpath through limiter 10. Upon the occurrence of a short circuitcondition within the electrical circuit which exceeds a predeterminedvalue, at least one of fuse elements 22 will melt or disintegrate, theheat generating by such melting or disintegration causing indicators 20to change state, as described above, and of course, preventing furthercurrent flow to the electrical equipment, from the electrical powersource, since the current path is broken.

[0032]FIG. 7 is an exploded view of a current limiter 100 in analternative embodiment of the present invention. Current limiter 100includes a base 102 and a cover 104. Base 102 includes a longitudinalslot 106 into which a multi-phase line-to-load insulating barrier 108 isinserted. Barrier 108 includes both a plurality of phase barriermounting slots 110 and a plurality of fuse element mounting notches 112.Phase barrier mounting slots 110 are used to engage phase barriers 114,which also include mounting notches 116 which engage barrier 108 asphase barriers are fitted onto barrier 108. Fuse element mountingnotches 112 engage mounting notches 118 on switch back fuse elements120.

[0033] Switch back fuse elements 120, as used herein refer to a fuseelement which includes first and second portions aligned with oneanother in a single plane and a third bridge portion joining an upperend of each of the first and second portions. In contrast to folded backfuse elements 22 (shown in FIGS. 1-6), switch back fuse elements aresubstantially planar fuse elements, yet, like folded back fuse elements22, are disposed in a compact area without reducing lineal length of thefuse elements. Further, fuse elements 120 include a first end 122 and asecond end 123 which make electrical contact with side-installedcontacts 124 and a bent flat member 126 at a right angle to the plane offuse element 120 which provides stiffness to the thin metal used inconstruction of fuse elements 120. Contacts 124 are L-shaped and includea bottom surface 128, an upper surface 130, and a vertical portion 132.

[0034] Contacts 124 are inserted into base 102 using contact mountingslots 134 which are fabricated into base 102. Contact mounting slots 134include a contact resting surface 136 onto which a bottom surface 128 ofcontacts 124 rests when contacts 124 are fitted into contact mountingslots 134. Contact mounting slots 134 further include an upper opening138 through which ends 122 and 123 of fuse elements 120 come to restupon upper surface 130 of contacts 124.

[0035] Cover 104 includes arc quenching media filling holes 140 intowhich, in an exemplary embodiment, quartz silica sand is inserted toabsorb arc energy when current limiter 100 opens or operates. Afterfilling, plugs 142 are inserted into filling holes 140. Cover 104further includes riveting holes 144 which align with riveting holes 146in base 102 when cover 104 is fitted onto base 102. In an exemplaryembodiment, rivets 148 are used to attach base 102 to cover 104 and areinserted into riveting holes 144 and 146 as is well known. Of courseother attachment methods may be employed to attach cover 104 to base 102including, but not limited to, ultrasonic welding processes and otherattachment means, for example, screws.

[0036]FIG. 8 is a partial assembly view of current limiter 100 andillustrates a physical relationship of base 102, multi-phaseline-to-load insulating barrier 108, phase barriers 114, and fuseelements 120. Further illustrated are first ends 122 and second ends 123(shown in FIG. 7) of fuse elements 120 contacting upper surfaces 130 ofcontacts 124. Ends 122 and 123 are, in alternative embodiments, solderedor welded to upper surfaces 130.

[0037]FIG. 9 is a partial assembly view of current limiter 100 whichillustrates that contact mounting slots 134, and therefore contacts 124for each fuse element 120 are offset from one another, that is, fromline to load, to provide an adequate space between contacts 124 forcompliance with various component regulatory agencies.

[0038]FIG. 10 is a view of a completely assembled current limiter 100,with cover 104 attached to base 102 using rivets 148. A chamber (notshown) defined by the space enclosed by cover 104 has been filled withan arc quenching media, for example, sand (not shown) using fillingholes 140 which have been subsequently plugged using plugs 142. Contacts124 are electrically connected to fuse elements 120 (shown in FIGS. 7-9)and are exposed and configured for insertion into an electrical circuit(not shown). Cover 104 is further configured with a plurality of notchesand indentations, collectively referred to herein as mounting features160, which are used for hooking or securing current limiter 100 into aninterface block (not shown) or other mounting devices (not shown) withinan electrical application.

[0039] Aside from the noted structural differences, current limiter 100operates similar to current limiter 10 as described above.

[0040] Current limiter 10 (shown in FIGS. 1-6) and current limiter 100(shown in FIGS. 7-10) provide an economical way to protect electricalcircuit from short circuits, and further allow circuit designers toincorporate circuit breakers for overload protection, which are smallerand much less expensive than known circuit breakers which can provideshort circuit protection. Further, short circuit current limiters 10 and100 are configurable for insertion into International ElectrotechnicalCommission (IEC) style motor starter and motor contactor installations,and in one particular embodiment, and denoted by dimension W in FIGS. 6and 10, are about 45 mm wide for a 30 Ampere rated current limiter.

[0041] While the embodiments described above pertain predominately toshort circuit only protection devices for use in combination withcircuit breaking devices, it is contemplated that aspects of the presentinvention could be applied generally to fused devices providingadditional fault current protection, including but not limited to fullrange fuse elements.

[0042] While the invention has been described in terms of variousspecific embodiments, those skilled in the art will recognize that theinvention can be practiced with modification within the spirit and scopeof the claims.

What is claimed is:
 1. A short circuit current limiter comprising: atleast one fuse element comprising a first end and a second end, saidfuse element incorporating one of a folded back configuration or aswitch back configuration; a plurality of contact terminals, one of saidcontact terminals electrically connected to each said end of each saidfuse element; a terminal base for mechanically mounting said fuseelements and said contact terminals; and a cover configured to engagesaid terminal base to protect said fuse elements.
 2. A short circuitcurrent limiter according to claim 1 comprising three fuse elements, andwherein said current limiter is about 45 mm in width.
 3. A short circuitcurrent limiter according to claim 1 further comprising a visualindicator to show a status of each of said fuse elements.
 4. A shortcircuit current limiter according to claim 1 wherein said visualindicators comprise a micro-crystalline organic wax applied to a blackcolor substrate.
 5. A short circuit current limiter according to claim 1wherein said cover and said terminal base are fabricated from athermoplastic.
 6. A short circuit current limiter according to claim 1further comprising at least one line-to-load insulating barrier for eachfuse element.
 7. A short circuit current limiter according to claim 6wherein the insulating barriers are fabricated from a fiberglasscomposite material.
 8. A short circuit current limiter according toclaim 1 wherein said contact terminals and said ends of said fuseelements are electrically connected by one of welding and soldering. 9.A short circuit current limiter according to claim 1 wherein said coveris configured with insulating phase-to-phase barriers to preventmultiple fuse elements from shorting to one another.
 10. A short circuitcurrent limiter according to claim 1 further comprising insulating phasebarriers inserted into phase barrier mounting slot in said base toprevent multiple fuse elements from shorting to one another.
 11. A shortcircuit current limiter according to claim 1 wherein said currentlimiter incorporates fuse elements with a switch back configuration,said fuse elements further comprising a bent flat member to providestiffness to said fuse element.
 12. A three pole current limitercomprising: three short fuse elements incorporating one of a folded backconfiguration or a switch back configuration, said fuse elements havinga first end and a second end; a plurality of contact terminals, one ofsaid contact terminals electrically connected to each said end of eachsaid fuse element; a terminal base for mechanically mounting said fuseelements and said contact terminals; and a cover configured to engagesaid terminal base to protect said fuse elements.
 13. A three polecurrent limiter according to claim 12 wherein said current limiter isabout 45 mm in width.
 14. A three pole current limiter according toclaim 12 further comprising a visual indicator to show a status of eachof said fuse elements.
 15. A three pole current limiter according toclaim 12 further comprising at least one line-to-load insulating barrierfor each fuse element.
 16. A three pole current limiter according toclaim 12 wherein said cover is fabricated with insulating phase-to-phasebarriers to prevent multiple fuse elements from shorting to one another.17. A three pole current limiter according to claim 12 furthercomprising insulating phase barriers inserted into a phase barriermounting slot in said base to prevent said fuse elements from shortingto one another.
 18. A current limiter comprising: a terminal base; atleast one pair of contact terminals attached to said base, each pair ofcontact terminals including contact portions extending on opposite sidesof said base; a cover configured for attachment to said base, at leastone of said base and said cover comprising a barrier defining acompartment; and a fuse element disposed within said compartment whensaid cover is attached to said base, said fuse element comprising afirst end and a second end, said fuse element engaged to said barrierand said first and second ends of said fuse element coupled to one ofsaid at least one pair of contact terminals.
 19. A current limiteraccording to claim 18 further comprising a fuse element indicatorresponsive to operation of said fuse element.
 20. A current limiter inaccordance with claim 18 further comprising three pairs of contactterminals and three fuse elements coupled to each said respective pairsof contact terminals.
 21. A current limiter comprising: at least oneshort circuit only fuse element, said fuse element comprising a firstportion and a second portion extending substantially parallel to oneanother, and a third portion extending substantially perpendicular toand joining said first and second portions; first and second contactterminals coupled to said at least one fuse element, said at least onefuse element extending between said contact terminals; a terminal basefor mechanically mounting said at least one fuse element and saidcontact terminals; a cover configured to engage said terminal base toprotect said at least one fuse element.
 22. A current limiter accordingto claim 21 further configured to be about 45 mm in width.
 23. A currentlimiter according to claim 21 further comprising at least one fuseelement indicator visible through said cover.
 24. A current limiteraccording to claim 21 comprising three short circuit only fuse elements.25. A current limiter comprising: at least one short circuit only fuseelement comprising a substantially planar first portion, a secondportion extending substantially parallel to said first portion, and athird bridge portion joining said first and second portions; first andsecond contact terminals coupled to said at least one fuse element, saidfuse element extending between said first and second contact terminals;a terminal base for mechanically mounting said fuse elements and saidcontact terminals; and a cover configured to engage said terminal baseto protect said fuse elements.
 26. A current limiter according to claim24 wherein said third portion of said fuse elements comprises a bentflat member at a right angle to the plane of said fuse element toprovide stiffness to said fuse element.
 27. A current limiter accordingto claim 24 wherein said at least one short circuit only fuse elementcomprises three short circuit only fuse elements.